The present invention relates to techniques for employing gas sensors for measuring concentration information for one or more gasses in a fluid environment.
Numerous gas sensors are known in the art. Gas concentrations can be measured by observing the changes in electrical properties (for example, resistance or capacitance) of the sensors. Thus, resistive sensors, capacitive sensors, and semiconductor sensors such as transistor, or diode sensors are known in the art
As a particular example, thin-film PdNi alloy resistors have been used to detect gasses such as hydrogen and measure its concentration. Hydrogen is soluble in PdNi and the resistivity of the thin-film PdNi alloy increases upon exposure to hydrogen and the amount of increase is proportional to the square root of hydrogen partial pressure. Some gas sensors, such as one that has a PdNi lattice, may have increased resistance or changed capacitance or a characteristic of the semiconductor as concentration of H2 increases. For example, resistance may increase linearly in proportion to the levels of hydrogen in a PdNi lattice, which in turn is related to gaseous H2 pressure as described by Sievert's law. This law holds that, at moderate pressures, concentration for hydrogen dissolved in solid metals is approximated by the following relationship:c≈s*p1/2 where c is the concentration of dissolved hydrogen in equilibrium with gaseous hydrogen at pressure p, and s is Sievert's parameter.
For a given application, a gas sensing system can be designed to detect the pressure of a target gas, for example, H2. In addition to the target gas pressure, however, there may be factors that can influence a gas sensor's measurements. For example, temperature of the gas sensor may influence measurements. To address this issue, a heater may be used to maintain the gas sensor within a desired temperature range. In addition to temperature, other factors may influence gas sensor measurements, such as a bias voltage applied to the gas sensor or the overall pressure of the fluid environment. These measurements are also prone to errors due to baseline drift associated with aging and the presence of unwanted gases, and shifts in the sensor calibration. One solution to this problem is to employ a system that performs calibrations and performs gas measurements at two different temperatures.
Non-target gasses, such as O2, may also influence a gas sensor's measurements. The presence of non-target gasses may influence or interfere with target gas measurements in at least two ways. First, because the sensor is responding to both a target and a non-target gas, the sensor reading may be too high or too low. In this respect, the influence of the non-target gas may be thought of as an offset to the target gas reading. Second, the presence of a non-target gas can alter the way a sensor measures a target gas. For example, non-target gasses can occupy receptor sites inside or on the surface of the lattice. This leaves less available receptor sites, thereby making the sensor less sensitive to the target-gas. As another example, in a PdNi gas sensor, the presence of oxygen in the lattice may affect the resistive or capacitive characteristics of the sensor. Thus, an oxygen-permeated lattice may respond to the presence of hydrogen in a different way than if the lattice was not permeated with oxygen. When oxygen permeates the lattice adsorption of hydrogen results in the formation of molecules such as H2O, OH, etc. These molecules may, by themselves, influence the resistive or capacitive characteristics of the gas sensor.
One attempted solution for reducing the influence of non-target gasses may be to use a blocking coating on a gas sensor to filter such non-target gasses. However, such a filter may reduce a gas sensor's sensitivity or response time. Another attempted solution may be to use multiple gas sensors to specifically detect non-target gasses to determine and account for concentration information for non-target gasses. However, such a solution may be expensive and/or introduce additional system complexity. Yet another attempted solution is to simply limit gas-sensing applications to ones that do not include interfering gasses. A solution to the drift problem is to repeatedly recalibrate a drifting sensor manually.
Therefore, there is a need for a new method for detecting hydrogen and measuring its concentration, which does not suffer from these disadvantages.